Using HF in TiO2 synthesis (left) changes the crystals’ morphologies and facets relative to an HF-free method (right), as seen in these TEM images, and may unexpectedly alter the crystals’ catalytic properties.

Credit: ACS Catal.

Using HF in TiO2 synthesis (left) changes the crystals’ morphologies and facets relative to an HF-free method (right), as seen in these TEM images, and may unexpectedly alter the crystals’ catalytic properties.

Credit: ACS Catal.

Titanium dioxide’s knack for mediating chemical reactions upon exposure to light has made the material a popular photocatalyst for various applications, such as sterilizing glass and other surfaces and splitting water to make hydrogen.

Researchers reported a few years ago that using hydrofluoric acid to synthesize nanocrystalline TiO2 significantly boosts its catalytic activity by exposing the material’s most active crystal faces. But a study just published in ACS Catalysis indicates the enhanced activity is mainly due to residual HF, not any particulars of crystal faceting (2013, DOI: 10.1021/cs400216a).

A crystal’s faces may be identical chemically, but structural and electronic differences can lead to differences in surface energy and catalytic activity. TiO2’s so-called (001) face, for example, is known to be more active than other TiO2 faces. For that reason, researchers commonly use HF to synthesize TiO2 because the acid yields a large fraction of (001)-faceted crystals.

Liqiang Jing of Heilongjiang University in China and coworkers confirmed via X-ray diffraction that increasing HF concentration during synthesis increases the fraction of crystals exhibiting (001) facets. And in degradation tests using acetaldehyde and phenol as model pollutants, the team confirmed that the most active TiO2 catalysts are indeed the ones with primarily exposed (001) faces. However, when the group removed surface fluoride—confirmed by surface analyses—catalytic activity plummeted despite the large fraction of (001) facets. The group proposes that surface-bound HF greatly enhances adsorption of O2 molecules, which capture electrons liberated by light and are thereby stimulated to react.

Can Li, a research director at China’s Dalian Institute of Chemical Physics and an expert in this area, says the work is convincing. And Baibiao Huang of Shandong University in China notes that the finding may be applicable to other oxide semiconductor photocatalysts with high-energy surfaces. “For that reason, it is of great significance,” he says.